1. Field of the Invention
The present invention relates to a method for balancing the motion of the movable masses of a bilinear electrodynamic motor.
The invention finds a particularly advantageous application in the field of alternating-cycle cryogenic machines, Stirling machines or pulsed-gas tubes, implementing bilinear electrodynamic motors with movable masses forming pistons, and more especially cryogenic machines intended to be carried onboard spacecraft such as Earth observation satellites. In this application, the bilinear electrodynamic motors play the role of compressor of the fluid used, helium for example.
2. Related Art
The operating principle of a bilinear electrodynamic motor is based on the generation, by induction coils, of cyclic magnetic forces which impart a rectilinear motion to the magnetized movable masses constituting the pistons of the motor and which are mounted on mechanical bearings which, on account of their construction, develop an axial elastic restoring force proportional to the displacement of the movable masses. The latter are therefore characterized by a mechanical resonant frequency determined by the mass in motion, the stiffness of the bearing, the magnetic stiffness and the fluidic loading.
The driving of the motor then consists in applying an excitation current to the induction coils at the mechanical resonant frequency of the magnetized movable masses, so as to obtain a natural amplification of the displacement motion of the pistons.
In bilinear electrodynamic compressors, the movable masses of the pistons are aligned in the same compression chamber and oscillate in mechanical opposition at the frequency of the coil excitation current, generally a sinusoidal current. This assemblage exhibits the advantage of a natural balancing of the movable masses in motion, which is not the case for single-piston linear compressors.
However, the tolerances on the mechanical and magnetic parameters, such as the mass, the mechanical and magnetic rigidity, the alignment, etc., lead to slightly different mechanical responses of the two half-motors for an identical electrical setpoint, and consequently induce vibrations of the motor along the axis of displacement of the movable masses of the pistons.
In an application to satellite-based Earth observation, this residual vibratory level leads to a degradation in image capturing, all the more so as the severe mechanical environment during launch in terms of vibrations and of impacts of the launcher, as well as the thermal environment in orbit excluding any thermal transfer by convection, demand that the compressor be fixed in a rigid manner on the structure of the satellite, thus promoting the propagation of the vibrations generated by the compressor towards the other equipment also fixed to the structure of the satellite, in particular the image capturing cameras.
Moreover, having regard to the required lifetimes (between 5 and 10 years), it is necessary to track the evolution of the balancing of the compressor so as to guarantee a minimum level of induced vibrations throughout the aging.
Current solutions for reducing the residual vibrations due to a defect in balancing the motion of the movable masses consist in optimizing the setpoint of the drive current for one of the movable masses with respect to the other, according to a master-slave system.
For this purpose, load sensors or accelerometers are placed in mechanical relation with the compressor so as to provide a measurement of the vibrations induced, on the compressor, by a possible imbalance between the displacements of the two pistons. The optimal setpoint of the drive current for the slave-piston is obtained when the vibration measurement obtained by the load sensors or the accelerometers is at a minimum.
The load sensors are piezo-electric washers placed at the mechanical interfaces for fixing the compressor with the structure of the satellite. Sensors of this type present a certain number of drawbacks, however.
First of all, while they are capable of measuring the residual vibrations specific to the compressor, the load sensors may also record those originating from other equipment fixed to the same mechanical structure of the satellite. The measurement of the vibrations sought is therefore disturbed by the mechanical environment of the compressor.
As piezo-electric sensors are poor thermal conductors, it is necessary to provide a different thermal path from the mechanical path passing through the washers to discharge the thermal dissipations of the compressor, namely the heat of compression of the gas, losses due to the Joule effect, to eddy currents, to hysteresis, etc. By way of example, an ad hoc thermal path can be achieved with conducting braids placed in short-circuit on the piezo-electric washers. This very obviously results in complex and more expensive integration.
Finally, it is very difficult to obtain a redundancy of these load sensors, considering their specific mechanical setup.
Likewise, the use of accelerometers disposed on the compressor does not lead to satisfactory results for the following reasons.
The measurement provided by the accelerometers generally exhibits a low signal-to-noise ratio on account of the significant masses on which the compressor is fixed. Moreover, the force transmitted is reconstructed by interpretation of the acceleration measurement according to an effective mass, the resultant of the movable masses, which is difficult to evaluate and therefore imprecise.
Just as for the load sensors, the acceleration measurement is disturbed by the mechanical environment around the compressor, thus leading to the measurement of accelerations which do not depend on the compressor.
In reality, the acceleration measurement is well adapted to a suspension mounting of the compressor and not to mounting on a rigid interface by bolting.
However, a traditional suspension mounting, necessary for correct measurement of the acceleration, decouples the structure from the interfaces and therefore imposes conditions that are hardly compatible with space applications, like the creation of a specific thermal path to discharge the heat dissipations and the installation of a mechanism for disabling the suspension, and then for re-enabling when the compressor has to support external mechanical loadings.
Finally, the accelerometers and their associated conditioning are expensive.